Which of the following is used to detect gamma rays?

Gamma rays are high-energy electromagnetic radiation and are challenging to detect due to their penetrating ability. A Geiger counter is specifically designed to detect ionizing radiation, including gamma rays. It operates by using a Geiger-Müller tube that registers the ionization caused by radiation passing through it. When gamma rays interact with the gas inside the tube, they create ions that lead to a measurable electrical pulse, which is then counted and displayed.

Other options may have some utility in detecting or measuring different forms of radiation but are not suitable for gamma ray detection. For instance, X-ray film and photographic film are primarily used for capturing images of X-rays rather than specifically detecting gamma rays. A photodiode is more commonly used to detect visible light and lower energy radiation rather than the high-energy gamma radiation. Therefore, the Geiger counter stands out as the most appropriate and commonly used instrument for detecting gamma rays.